fix(storage): honour VOUCH_KB_PATH env var in discover_root

[plind-junior]

the var was already documented in adapters/generic-mcp/README ('force a specific KB root, skip discovery') but never wired into the code. now discover_root() reads VOUCH_KB_PATH first and short-circuits the upward walk, with a clear error if the value isn't an existing .vouch directory.

closes the doc-vs-code drift and removes the per-host 'cwd: ...' ceremony needed by Claude Desktop (and any other host that launches the stdio server from a default working directory).

What changed

Why

What might break

VEP

Tests

• make check passes locally (lint + mypy + pytest)
• New / changed behaviour has a test
• CHANGELOG.md updated under ## [Unreleased]

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